A linear ion trap includes four electrodes and operates with an asymmetrical trapping field in which the center of the trapping field is displaced from a geometrical center of the trap structure. The asymmetrical trapping field can include a main AC potential providing a quadrupole component and an additional AC potential. The main AC potential is applied between opposing pairs of electrodes and the additional AC potential is applied across one pair of electrodes. The additional AC potential can add a dipole component for rendering the trapping field asymmetrical. The additional AC potential can also add a hexapole component used for nonlinear resonance. A supplementary AC potential can be applied across the same pair of electrodes as the additional AC potential to enhance resonant excitation. The operating point for ejection can be set such that a pure resonance condition can be used to increase the amplitude of ion oscillation preferentially in one direction. Ions trapped in the composite field can be mass-selectively ejected in a single direction to an aperture in one of the electrodes.